I agree with Bill on backing out the low-limit screw very slightly. You want to give it just enough travel so it can drop the chain into the granny ring without throwing it off the side. Trial and error...

But I'll mention that chainrings don't like to "give up" the chain under load, especially when you're grinding uphill and running out of gears in the cassette, so it helps to plan ahead and get into the granny when the incline is still reasonable. And when it's too late to shift into the granny ring...


(Jobst Brandt pic from pardo.net)

- Scott